A (meth)acrylic polymer having a substituent at one molecular end can be used as a starting material for a block polymer or graft polymer.
Here, as a method for producing a (meth)acrylic polymer having a substituent at one molecular end, for example, a method of subjecting a (meth)acrylic monomer to a polymerization reaction by using, as an initiating species, a compound having a thiol and a functional group as described in each of Patent Literature 1 and Patent Literature 2, and a method in which a polymerization reaction is carried out by using a halide as an initiating species, and then the halogen is substituted with a substituent as described in Patent Literature 3 are known.
In particular, in the method described in each of Patent Literature 1 and Patent Literature 2, which uses a bulk polymerization reaction, since the reaction proceeds gently, reaction control is easy, and the method is suitable for mass production, which is advantageous.
In the bulk polymerization process such as the one described above, formerly, in some cases, the reaction was carried out by introducing the air as shown in Patent Literature 4. The air contains about 20% by volume of oxygen, and because of the presence of the oxygen, reaction control is complicated, the disulfide content is increased, and transparency, which is a feature of acryl, is impaired by a side reaction of the thiol compound and the inhibitor in the monomer. For these reasons, usually, as shown in Patent Literature 5 or 6, a polymerization reaction is carried out after the air inside a reaction vessel has been displaced with a gas, such as nitrogen gas, argon gas, or carbon dioxide gas, that does not affect the polymerization reaction. That is, in each of Patent Literature 5 and Patent Literature 6, after the air inside the reaction vessel is sufficiently displaced with nitrogen gas, a polymerization reaction is carried out.
By carrying out a polymerization reaction in an inert gas atmosphere in such a manner, it was considered that the compound having a functional group and a thiol group, used as an initiating species, stably functioned as the initiating species.
However, it has become clear that the (meth)acrylic polymer produced by such a method has low polymerization stability, and when a block polymer or graft polymer is produced, the blocking ratio or grafting ratio tends to be low.
As a result of careful studies, the present inventors have found that this is caused by the fact that by carrying out a polymerization reaction in an inert gas atmosphere, the substituent introduction ratio decreases.